As shown in FIG. 1, many prior art electrical power systems 100 use a brushless, synchronous electrical starter/generator 110 to generate AC power. Commonly, the starter/generator 110 includes a main generator, an exciter generator and a rectifier assembly 113 mounted on a rotor 112. The main generator includes a main stator 116 having a main stator coil (polyphase AC stator winding) and the DC main field winding 115. The exciter generator includes an exciter stator 114 including a DC winding 120 and the polyphase AC exciter armature winding 111. Rotor 112 includes a DC main field winding 115, polyphase AC exciter armature winding 111 and the rectifier assembly 113. For aircraft engine applications, the rotor 112 may be driven by an aircraft engine (not shown), after engine starting, to develop electrical power in the main stator coil 116. The electrical voltage output from main stator coil 116 is regulated at a point of regulation (POR) 108 for delivery to aircraft loads using an AC bus 118. In an exemplary embodiment, when DC excitation is supplied to DC winding 120, rotation of the generator shaft (not shown) by the aircraft engine causes the generation of a polyphase (as shown in FIG. 1) or single-phase voltage in the armature winding 111 that is rectified by the rectifier assembly 113 and coupled to the winding 115. This rectified voltage sets up a DC field in the main rotor field 115 which causes a rotating magnetic field in the main stator coil 116 that produces output power with regulated voltage at POR 108 (prior to the bus contact switch) for delivery to AC bus 118.
Additionally, the system 100 may use the starter/generator 110 as a motor to start the aircraft engine. An external power source (exciter power supply—EXPS) 104 is coupled to the generator 110 using the exciter stator 114. The coupled power from EXPS 104 induces AC power through transformer effect in the polyphase winding 111 or single-phase (not shown) of the rotor 112 because no relative motion between rotor and stator exists at zero speed. The AC power established in winding 111 may be rectified by rectifier assembly 113 to generate DC power in the main field winding 115. Additionally, a start converter 106 is used to supply controlled AC power to main stator coil 116 such that sufficient torque is produced by the starter/generator 110. This torque is produced by the interaction between the flux in the main rotor winding 115 and the current (flux) established in coil 116. The frequency of the controlled AC power is increased from 0 Hz (0 RPM) to a predetermined frequency corresponding to the angular speed of the for starter/generator 110 at the end of start. The phase of the current for the supplied AC power input is controlled to develop the desired torque for starter/generator 110. Advantageously, the current is approximately 90 degrees ahead of the flux established in winding 115 where this torque causes the generator shaft to rotate the aircraft engine, start it, and bring it to a predetermined (rated) speed.
Conventionally, after engine start using the exciter power supply 104 and start converter 106, control switches to a separate unit (generator control unit—GCU) 102 to supply DC power to the generator 110 and deliver regulated voltage to the AC bus 118 via the POR 108. Thus, two separate units 102, 104 must be utilized to provide control and input power, using a complex switching unit 103, for both starter and generation functionality for the electrical power system which leads to complex, costly, and heavy installation for the system in the aircraft.
Therefore, due to the disadvantages of current electrical power systems, there is a need to provide an aircraft electrical power system that supplies both starting and generating functionality using a single control/power unit which reduces the cost and weight of the system installation in the aircraft.